The present invention relates to an optical fiber cable for indoor and outdoor applications, and more particularly, an optical fiber cable having an optical fiber subunit for independent use indoors or within small splice enclosures and a support subunit which makes the cable suitable for outdoor use and is easily separable from the optical fiber subunit.
Optical fiber is now used in a variety of telecommunications applications because of its small physical size and high bandwidth capacity.
The use of optical fibers in telecommunications applications initially involved the application of optical fiber cables, which were constructed with a large number of optical fibers, as long distance telecommunication data links between a central office and a switching office and between different switching offices of telecommunications data service providers, e.g., a telephone company. Data transmission links installed outdoors and extending to the premises of a customer of a telecommunications data service provider from, for example, a switching office or an office in a local distribution network, which are known as drop cables and primarily constituted copper cables, however, usually were not replaced by optical fiber links.
Telecommunications customers have begun to demand increased bandwidth for their offices or homes. Consequently, optical fiber links have begun to be deployed further into the telecommunications infrastructure to satisfy such demand. For example, optical fiber links have become commonplace in city or metropolitan area optical fiber ring networks and also in the main data distribution lines between a switching office and a customer""s home or office.
It is further envisioned that some telecommunications networks will be constructed to include optical fiber links which extend to within the customer""s premises from, for example, an optical ring switching office. Such networks often are referred to as xe2x80x9cFiber To The Homexe2x80x9d or FTTH networks. Therefore, in a FTTH network, even the drop cables would constitute optical fiber links.
A drop cable frequently extends from a pole to the customer""s premises. The cable may also extend to such pole from a terminal or connection box located hundreds of feet from the pole, and the cable may be suspended between poles hundreds of feet apart. Thus, a satisfactory drop cable must withstand outdoor weather conditions and be relatively robust for such use. In addition, the cable must have a relatively long life when exposed to sunlight, particularly, ultra-violet light.
The optical fiber of the drop cable may or may not extend into the customer""s premises. The drop cable can terminate at, or close to, the entrance to the customer""s premises, but in such case, and if the optical fiber is to extend into the customer""s premises, it is then necessary to splice another optical fiber cable to the drop cable, usually at a connection box. Preferably, the jacket of the drop cable is flame retardant even when it is outdoors to prevent the spread of fire by way of the drop cable. The other cable which extends from the drop cable to the customer""s equipment is indoors and must meet some requirements different from the outdoor drop cable. For example, like the drop cable, the indoor cable should have a flame retardant jacket, and in addition, should be suitable for feeding through relatively small ducts, be flexible and lightweight and be capable of being bent to a relatively small radius.
A drop cable which can be used both outdoors and indoors is desirable not only because of the types of cables needed can be reduced, but also because a cable splice at or near the entrance to the customer""s premises can be eliminated. Ideally, a drop cable should be lightweight, inexpensive, flexible and self-supporting. It should not require any new hardware for installation, and should not require any new or special tools in order for the craft-person to install it. It should be designed so that fiber optical connectors can easily be installed on it, either in the field, or in a manufacturing setting. It should also be designed so that it can be routed within small termination enclosures so that the length of exposed fiber is minimized.
Cables with optical fibers and strength members which can be used as aerial cables are known in the art. See, for example, U.S. Pat. Nos. 4,763,983; 4,852,965 and 5,095,176. However, such cables are not suitable for both outdoor and indoor use.
Telecommunications data service provider customers expect continuous, uninterrupted, high quality data transmission service. An optical fiber, however, is a mechanically fragile structure whose optical signal transmission characteristics can degrade substantially if the fiber is mechanically stressed. Hence, an optical fiber drop cable can become non-functional, i.e., have too much optical signal attenuation for purposes of satisfactory optical signal transmission in a telecommunications application, if the cable design does not sufficiently limit or avoid severe mechanical stress on the fiber contained in the cable for the planned applications.
The planned application of an optical fiber drop cable outdoors, and in particular in a geographical region experiencing winter weather conditions, is likely to subject the fiber contained within the cable to additional loading. For example, an optical fiber contained in an optical fiber drop cable which is suspended in air from vertical supports is likely to experience additional stress and strain upon accumulation of a layer of ice on external surfaces of the cable. The resultant increase in the total weight or load on the cable caused by the ice layer would be a function of the radial thickness of the layer of ice and the outer diameter of the external surface of the cable. If the cable does not include sufficient load absorbing or distributing means, such as strength members, too much stress and strain likely would be placed on the fiber, thereby causing significant and unacceptable optical signal transmission attenuation.
There is a need for an optical fiber cable design available which can adequately satisfy customer and industry needs and demands for an optical fiber drop cable which provides reliable and high quality data transmission service in an aerial application in an outdoor environment and, simultaneously, provides ease of application in an indoor environment using standard hardware and equipment and satisfies indoor cable requirements.
Some prior art optical fiber cables include at least one strength member or layer of strength members to control the behavior of the drop cable when it is subjected to bending and, thus, protect the optical fibers within the cable from experiencing too much stress or strain. The inclusion of too large or multiple strength members within a cable, however, is disadvantageous in a FTTH application for several reasons. First, the inclusion of a plurality of strength members in the cable jacket is likely to make the cable extremely stiff. An overly stiff cable makes handling and maneuverability of the cable difficult because substantial energy would be required to bend the cable, which may be required during application of the cable indoors. Also, the inclusion of multiple strength members in the jacket, for example, disposed symmetrically about the optical fiber transmission media, greatly increases the outer diameter of the cable. As explained above, smaller cable outer diameter is desirable to decrease the potential loading that a layer of ice formed on an optical fiber drop cable which has been installed suspended from vertical poles can cause. Finally, it is more difficult to secure aerial hardware to multiple strength members than to a single strength member in an aerial application of a cable.
Indoor optical fiber cables, which are often referred to as premises cables, are available in a variety of forms which provide for flexibility, ease of maneuverability and ease of connectorization to standard hardware using standard tools. For example, so-called Simplex cables include only one fiber and are standardized to specific dimensions for which a wide variety of hardware is readily available. Another cable design, called a xe2x80x9czipcordxe2x80x9d, includes two Simplex cables of standard dimensions, which have been joined by a web and are easily separable for termination. Both cable designs tend to be very flexible and provide for ease of access to the fiber(s) contained within the cable. Current designs of optical fiber drop cables which include at least one support means having very high tensile strength in anticipation of planned outdoor aerial application, however, do not have the flexibility to provide for relative ease of application of the optical fiber components in an indoor environment or within small splice enclosures.
Therefore, there exists a need for an optical fiber drop cable which is inexpensive to manufacture, which is self-supporting and easily installed in an outdoor environment and which is sufficiently lightweight, compact and flexible and does not require any new hardware or new or special tools for application of the optical fiber transmission media contained within the cable in an indoor environment or within small splice enclosures.
The optical fiber cable of the invention has a jacket of flame retardant and ultra violet stabilized plastic and meets the requirements for both outdoor and indoor use. The jacket has two longitudinal portions interconnected by an intermediate longitudinal portion of a thickness less than the thickness of the two portions. One of the two portions contains a longitudinally extending strength member of sufficient tensile strength to support the cable when the cable is suspended outdoors between relatively widely spaced supports. The other of the two portions has a longitudinally extending bore which contains at least one tightly buffered, longitudinally extending optical fiber and can also contain a flexible, longitudinally extending strength member.
When the cable is suspended outdoors between supports, the intermediate jacket portion has sufficient strength to prevent separation of the strength member portion from the optical fiber portion, but when desired, the strength member portion can be separated from the optical fiber portion by severing the intermediate portion longitudinally to thereby form a strength member subunit and an optical fiber subunit. The optical fiber subunit is flexible and meets indoor riser requirements and can be fed through ducts, etc., without the strength member subunit, to the customer""s equipment. Since the optical fiber is tightly buffered, the fiber is protected when the optical fiber subunit is used alone, and the fiber can be additionally protected by one or more flexible strength members in the bore of the optical fiber subunit.
The strength member subunit normally will terminate near the entrance to the customer""s premises and unused strength member submit can be discarded. However, outdoors the strength member itself preferably is used to suspend the cable from supports. For this purpose, the jacket can be cut open at the strength member to permit separation of a length of the strength member from the jacket. Also, to facilitate such separation, the strength member is not bonded to the jacket.
In a preferred embodiment, the intermediate portion of the jacket is a web which extends along the longitudinal length of the cable and between the subunits and, at least in part, couples the fiber subunit to the strength member subunit.
In a preferred embodiment, each of the subunits is in the form of a cylindrical element which is circular in cross-section and which extends longitudinally along an axis which is parallel to the axis of the other subunit, and when the subunits are coupled to each other, the cable configuration has, in cross-section, the shape of the FIG. 8.
In a preferred embodiment, the fiber subunit has a longitudinal bore larger than the diameter of the tightly buffered fiber which is received therein, so that the fiber can move freely within the fiber subunit.
In a further preferred embodiment, the longitudinal bore of the fiber subunit includes at least one longitudinally extending strength member, such as a flexible aramid yarn, disposed about the fiber, without enclosing the fiber, to provide tensile strength to the fiber subunit and cushioning the fiber, especially when the fiber subunit is separated from the strength member subunit.